Surface Chemical and Physical Behavior of Chrysotile Asbestos in Natural Waters and Water Treatment

Citation

Bales, Roger Curtis (1985) Surface Chemical and Physical Behavior of Chrysotile Asbestos in Natural Waters and Water Treatment. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4q42-z892. https://resolver.caltech.edu/CaltechTHESIS:09292010-075726877

Abstract

Chrysotile asbestos fibers enter California waters from physical weathering of magnesium-silicate, serpentine rocks in mountains of the northern and central portions of the state. Chrysotile particles, initially positively charged below pH 8.9 because of their magnesium-hydroxide surface, become negatively charged due to dissolution and adsorption of organic matter. Magnesium release from chrysotile dissolving in 0.1 M inorganic electrolyte at pH 7-10 for up to five days is in excess of the 3:2 Mg:Si to silica molar ratio in the solid. The magnesium release rate exhibits a fractional dependence on hydrogen-ion concentration:

r = k₁'[H⁺]^(0.24)

The observed rate constant, k₁', depends on dissolution mechanism, specific surface area of the solid and charge-potential relation at the surface. In terms of a surface site-binding model, the fractional dependence implies that dissolution is limited by a chemical reaction involving less than one adsorbed proton per magnesium ion released. Silica release shows no clear pH dependence.

The rate of magnesium release is independent of the anions NO₃^-, Cl^-, SO₄^2-, HCO₃^-, oxalate or catechol. Oxalate inhibited and catechol slightly enhanced silica release at pH 7.5-8.5; other anions had no systematic effect. Chrysotile's dissolution rate (10^-15.7 mol/cm²•s at pH 8) is consistent with observations on other magnesium silicates and brucite.

Catechol adsorption onto chrysotile or aluminum oxide (pH 7.5-8.5) does not reach equilibrium but increases over five days. After one day the maximum adsorption density (Langmuir adsorption equation) on chrysotile is 0.7 x 10^-9 mol/cm² (50 x 10^-6 mg C/cm²), approximately one-third of the estimated density for proton exchange. The maximum adsorption density for natural organic matter was near 30 x 10^-6 mg C/cm² on both chrysotile and aluminum oxide.

Chrysotile adsorbs sufficient natural organic matter within one day to reverse its surface charge. The extent of reversal is larger than observed on aluminum oxide, because of selective dissolution of chrysotile's outer magnesium-hydroxide layer.

In reservoirs, submicron-sized chrysotile particles coagulate with larger (> 2 µm), negatively-charged particles that subsequently settle out. The rate at which freshly-suspended, positively-charged fibers coagulate is at least ten-fold greater than the rate for aged, negatively-charged fibers coagulate. Capture of chrysotile particles in water filtration is enhanced 10-fold or more by incorporating fibers into larger flocs.

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